The surface tension of the lung decreases markedly on compression resulting from deflation, as calculated from P-V data, and follows a force-area path very similar to that of several mucus surfaces. With changes in surface area of less than 50%, lung extract and mucus bubble surfaces are mechanically reversible. For decrease in area greater than 50%, surface tension approaches a lower limiting tension of 10–15 d/cm. On re-expansion of the surface, an upper limiting tension of 40–50 d/cm is approached. Hysteresis of considerable magnitude occurs and indicates a major alteration of the surface materials compressed beyond 50%. Estimates of lung surface area based on these considerations accord with histologic estimates. Submitted on December 4, 1958